Embodiments relate generally to an optical imaging system and, in particular, to short-wavelength infrared (“SWIR”) and mid-wavelength infrared (“MWIR” and collectively “SWIR-MWIR”) reimager.
Optical reimagers are commonly used in infrared optical systems and can be formed from a wide variety of optical elements depending on system design constraints. Optical reimagers are used to form an intermediate image, and then translate an intermediate image into a final image. Cooled infrared detectors are commonly used in high-performance infrared systems that require high responsivity and low noise. These detectors typically have a cold shield with an aperture, commonly referred to as the cold stop which is provided to limit the solid angle of radiation detected by the detector to the solid angle of light focused by the optical system. The cold-shield efficiency of an infrared system that uses cooled infrared detectors is the ratio of the solid angle of the light focused by the lens to the solid angle subtended by the cold stop. Optimally, an infrared optical system has 100 percent cold-shield efficiency.
Reimaging optical systems are commonly used as a means of pupil control for infrared optical systems that have cooled detectors where the cold shield diameter of the infrared system is used as the aperture stop of the system. When multiple optical systems are combined into one larger optical system, reimaging optical systems are commonly used to create a real pupil position at the interface of two optical systems. Using reimaging optical systems to relay the image in a single wavelength band of the cold stop to the entrance aperture of the entire system in order to minimize the size and weight of larger aperture objective lenses is well known to those skilled in the art.
A design goal for any high-resolution imaging optical system is to achieve diffraction-limited performance over a field of view while minimizing aberrations that adversely affect image clarity. To achieve diffraction-limited performance, optical systems are configured to correct, or compensate for, various optical aberrations such as spherical, coma, astigmatism, distortion, and chromatic.
A reimager may be created by combining a relay (e.g., two or more mirrors or a lens group) with imaging optical elements. Reimagers typically contain multiple optical elements to correct aberrations and achieve desired diffraction-limited performance. However current reimagers are designed to work well within a defined wavelength band (e.g., visible, mid-wave infrared (MWIR), long-wave infrared (LWIR), etc.).
The type of optical element materials used in reimaging devices depends on the wavelength band over which the optical system is working and the types of aberration being corrected. For example, chromatic aberration occurs when a lens fails to image all colors at the same focal point. This aberration arises because different lens materials have different refractive indices for different wavelengths of light.
Many types of optical materials have been developed to facilitate the reduction of chromatic aberrations over a certain range of wavelengths. Diffractive optical elements also may be used. By combining two or more lenses of different composition, the degree of aberration correction may be increased. Accordingly, there are many available tools to enhance the performance of an optical reimager, however, considerable shortcomings remain.
One such shortcoming is that currently the degree of aberration correction is limited to a specific wavelength or wavelength region. For example, it may be desirable to correct short-wavelength infrared (SWIR) and mid-wavelength infrared (MWIR) chromatic aberrations. Currently such corrections require at least two optical paths with SWIR chromatic aberrations corrected in a first path and MWIR chromatic aberrations corrected in a second path. Two or more paths have been required because different types of materials are used to correct chromatic aberrations over the different wave bands. More specifically, a strictly SWIR (e.g., 1.0 to 1.6 μm wavelength) optical system is typically color-corrected with optical glass, which transmits well out to approximately 2 μm wavelength. However, a strictly MWIR (e.g., 3.0 to 5.0 μm wavelength) optical system is typically color-corrected with a limited number of infrared materials such as silicon, germanium, zinc selenide, and zinc sulfide. In other words, glass does not transmit MWIR and many commonly used MWIR materials do not transmit SWIR. Therefore, the materials commonly used to chromatically correct optics in the SWIR waveband are significantly different from the common materials used to chromatically correct optics in the MWIR waveband.
Requiring the use of multiple optical paths adds complexity to optical systems that are used for imaging at different wavelengths. Therefore manufacturers and users of such optical systems would benefit from a system which was not as complex.